This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to the use of view angle tilting (VAT) readout for reducing MRI signal distortion from an object in the presence of magnetic field inhomogeneity including inhomogeneity due to the presence of metal in the object.
Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials and for medical imaging. It is generally non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. the radio frequency signals resulting from the precession of the spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are then combined to produce a volumetric image of the nuclear spin density of the body.
MRI signals for recording an image of an object are obtained by placing the object in a magnetic field, applying magnetic gradients for slice selection, applying a magnetic excitation pulse to tilt nuclei spins in the desired slice, and then detecting MRI signals emitted from the tilted nuclei spins.
The use of view angle tilting to reduce adverse imaging effects from magnetic field inhomogeneities is described by Cho et al. in “Total Inhomogeneity Correction Including Chemical Shifts and Susceptibility by View Angle Tilting” Med. Phys. 15(1), January/February 1988. The method uses a gradient applied on the slice select axis during readout, with an amplitude equal to that of the slice select gradient. The slice is then effectively viewed at an angle θ=tan−1(GZ/GX). When viewed at this angle, shifts in the slice selection plane compensate for shifts during readout, such that in-plane shifts are re-registered in the images.
The VAT technique has been employed also to improve depiction of biopsy needles in RF refocused MRI. See Butts et al. “Management of Biopsy Needle Artifacts: Techniques for RF refocused MRI”, Journal of Magnetic Resonance Imaging, 9:586–595 (1999).
However, view angle tilting images have suffered from an apparent blurring in the frequency encode direction that has hindered the widespread use of the technique. Typical spin echo and view angle tilting images are shown in FIG. 1. The arrowhead appearance of the needle artifact is eliminated with view angle tilting, but the VAT image suffers from blurring in the frequency encoding direction.
One possible cause of the blurring with VAT MRI is due to viewing the slice at an angle. This is because edges that are oriented exactly through-plane suffer from partial voluming effects when viewed at an angle.